I have found the cause of all this. If the OR clause is in there, this causes the problem. As soon as I remove the OR clause, everything is ok.

So, this takes 56 seconds:

SELECT TOP 101 Client.Numero,Client.Name,Client.Street
 FROM Client
 WHERE (Client.Name LIKE '242401%' OR Client.Altern_Name LIKE '242401%') AND Client.NoInvoice=3
 ORDER BY Client.Name

And, this is instant:

SELECT TOP 101 Client.Numero,Client.Name,Client.Street
 FROM Client
 WHERE Client.Name LIKE '242401%' AND Client.NoInvoice=3
 ORDER BY Client.Name

And, I double checked one more time. I do have index on Client.Name and Client.Altern_Name. I would say this is the only place in our application that I have this. This is why I never experienced it before.